Very Informational thread.. I currently just barely installed a 4 channel amp running all 4 doors, and newly Just started having this whine problem.
Today while idle it was blaring, and hitting gas even when in park it gets louder, but turning volume up even when playing nothing doesnt increase it.
I tried turning off my HU and the whine stopped. This makes me think its either the 4 channel amps ground, the HU needs a better whine (but never noticed it before and its the same HU from when I only amped the sub) or its the RCA's bit I got nice ones. Any suggestions? I think i'll try tapping into and adding a better ground for the HU, and rechecking the amps ground